The use of electrical measurements in prior art downhole applications, such as logging while drilling (LWD), measurement while drilling (MWD), and wireline logging applications is well known. Such techniques may be utilized to determine a subterranean formation resistivity, which, along with formation porosity measurements, is often used to indicate the presence of hydrocarbons in the formation. For example, it is known in the art that porous formations having a high electrical resistivity often contain hydrocarbons, such as crude oil, while porous formations having a low electrical resistivity are often water saturated. It will be appreciated that the terms resistivity and conductivity are often used interchangeably in the art. Those of ordinary skill in the art will readily recognize that these quantities are reciprocals and that one may be converted to the other via simple mathematical calculations. Mention of one or the other herein is for convenience of description, and is not intended in a limiting sense.
Directional resistivity measurements are also commonly utilized to provide information about remote geological features (e.g., remote beds, bed boundaries, and/or fluid contacts) not intercepted by the measurement tool. Such information includes, for example, the distance from and direction to the remote feature. In geosteering applications, directional resistivity measurements may be utilized in making steering decisions for subsequent drilling of the borehole. For example, an essentially horizontal section of a borehole may be routed through a thin oil bearing layer. Due to the dips and faults that may occur in the various layers that make up the strata, the distance between a bed boundary and the drill bit may be subject to change during drilling. Real-time distance and direction measurements may enable the operator to adjust the drilling course so as to maintain the bit at some predetermined distance from the boundary layer. Directional resistivity measurements also enable valuable geological information to be estimated, for example, including the dip and strike angles of the boundary as well as the vertical and horizontal conductivities of the formation.
Methods are known in the art for making directional LWD measurements. For example, LWD directional resistivity tools commonly measure or estimate a magnetic cross-component (e.g., the Hzx component) of the electromagnetic radiation as the tool rotates in the borehole (e.g., during drilling). Various tool configurations are known in the art for measuring such cross components. For example, U.S. Pat. No. 6,181,138 to Hagiwara teaches a method that employs an axial transmitter antenna and three co-located, circumferentially offset tilted receiver antennae. U.S. Pat. No. 6,969,994 to Minerbo et al., U.S. Pat. No. 7,202,670 to Omeragic et al., and U.S. Pat. No. 7,382,135 to Li et al teach a method that employs an axial transmitter antenna and two axially spaced tilted receiver antennae. The receiver antennae are further circumferentially offset from one another by an angle of 180 degrees. U.S. Pat. Nos. 6,476,609, 6,911,824, 7,019,528, 7,138,803, and 7,265,552 to Bittar teach a method that employs an axial transmitter antenna and two axially spaced tilted receiver antennae in which the tilted antennae are tilted in the same direction. U.S. Pat. Nos. 7,057,392 and 7,414,407 to Wang et al teach a method that employs an axial transmitter antenna and two longitudinally spaced transverse receiver antennae.
As is known to those of ordinary skill in the art, electrically anisotropic reservoir formations are commonly encountered during drilling. Directional resistivity measurements are sensitive not only to remote geological features such as bed boundaries, but also to the electrical properties of an electrically anisotropic near-bed (the bed in which the measurement tool resides). In particular, a homogeneous, electrically anisotropic near-bed often produces directional resistivity measurements that are similar to that of a remote geological feature. This “anisotropy effect”, if not properly accommodated can interfere with the use of directional resistivity measurements in the detection and/or characterization of a remote target. Likewise, the presence of a remote geological feature can also interfere with the proper characterization of the electrical properties of the near-bed (e.g., the determination of the vertical and horizontal conductivities of the formation).
One known technique for removing the above-described near-bed anisotropy effect involves the use of a pair of transmitter antennae deployed axially symmetrically about a receiver or receiver pair. The transmitters are typically fired sequentially. One of the received signals is then subtracted from the other in order to reduce the anisotropy effect. While this technique may be commercially serviceable, there is a need for further improvement.